In digital communication systems, since data is generally transmitted over a limited band, interference is occurred in adjacent symbols due to a time dispersion effect that allows pulse energy of symbols to be dispersed into adjacent symbol pulses. Besides, transmitted data is affected by a variety of channel distortions. This channel distortion phenomenon includes multi-path phenomenon, frequency offset, phase jitter and so on. This phenomenon causes InterSymbol Interference (ISI), implying that transmission symbols affect adjacent symbols in digital communication systems, which becomes a great obstacle in obtaining required data.
In particular, under Single Frequency Network (SFN) environment, there exists a boundary area where signals of diverse transmitters transmitted at a same frequency coexist with each other. Because the magnitude of signals from each transmitter is very similar in the boundary area, a big ghost may be occurred. Further, a ghost with very large time delay may be issued in the boundary area since the signals are simultaneously received from near and far transmitters. Due to issuance of such a ghost, there may be many cases that raise ISI most largely.
In order to prevent the above problem, a conventional receiver (digital broadcasting receiver) employs a channel equalizer to decrease symbol errors caused by ISI.
In communication channel, distortion factors as discussed above may be variable or fixable according to receiver circumstance. Typically, the digital broadcasting receivers mainly adopt an adaptive equalizer which adaptively updates tap coefficients according to time.
Now, a description will be given on a configuration of a conventional channel equalization device with reference to FIG. 1.
Especially, FIG. 1 illustrates a configuration of a general Decision Feedback Equalization (DFE) device.
As illustrated therein, in the general DFE device, a digital filter 11 removes ISI components that introduce distortions in a baseband signal received by a receiver (digital broadcasting receiver). At a symbol detector (simple quantizer) 12, a signal from the digital filter 11 is compared with a preset threshold to produce decision data.
Inputs to a tap coefficient updater 13 are an output signal of an equalizer input signal storage unit 17, an output signal of the digital filter 11, and error data selected by a switch 16, wherein an error is computed to update tap coefficients of the digital filter 11.
At a training sequence storage unit 14, a training data sequence that is also known by a transmitter (digital broadcasting transmitter) is stored therein. This training data sequence is read out in a training mode and provided to the tap coefficient updater 13.
At a statistical data calculator 15, a statistical error is calculated in a blind mode and forwarded to the tap coefficient updater 13.
At the switch 16, one of the outputs from the training sequence storage unit 14, the statistical data calculator 15 and the symbol detector 12 is selected in response to a selected mode and provided to the tap coefficient updater 13 as error data.
Then, at the tap coefficient updater 13, a corresponding error signal is derived; and data corresponding to the tap coefficients of the digital filter 11 is read out from the equalizer input signal storage unit 17 to update the tap coefficients. The updated tap coefficients are then delivered to the digital filter 11.
As the channel equalization device, this DFE device is widely used in digital broadcasting receivers. Typically, the DFE device has a structure that eye diagram of its output is open, which serves to precisely and easily make output signal decision as performance decision factor of the equalization device. Therefore, if an output of the symbol detector is a correctly decided symbol, a feedback filter is often utilized in the digital broadcasting receivers since there is no problem such as noise amplification phenomenon at output of the filter caused by a linear equalizer during the channel equalization while removing ISI by a previously decided symbol.
However, there exist many cases that it fails to open the eye diagram of the filter output under a multi-path environment having a ghost with a long time delay and with a size similar to that of a signal from a main path, like the SFN environment. If the eye diagram is not open, there is a very high possibility that raises decision error in the symbol detector. This brings about an error propagation problem that allows error decision to be accumulated through the feedback loop of the DFE apparatus.
To equalize communication channel under such a poor environment, there is a proposed method which changes channel characteristic of a received signal with a channel matched filter, wherein some degree of good performance is obtained, compared with the existing equalization device. However, this method also causes error propagation and fails to conduct stable equalization once decision error is taken place. This is disclosed in Y. Wu's proposal, entitled “An ATSC DTV Receiver with Improved Robustness to Multipath and Distributed Transmission Environments”, IEEE Trans. Broadcasting, vol. 50, no. 1, pp. 32-41, March 2004.
The channel equalization method proposed by Y. Wu performs channel estimation by using over-sampled data and then provides a channel matched filter from the estimated channel information, wherein a fractionally-spaced equalization apparatus is used. However, since this method carries out over-sampling and fractionally-spaced equalization, the degree of complexity is very high. In addition, the method employs a simple quantizer (slicer) as symbol detector, which yields an error propagation problem by decision error.
Consequently, there has been a need for development of a new equalization device that can perform a more stable equalization while having low degree of complexity.